I just got this from the American Lung Association:
6 Main Factors that contribute to outdoor air pollution:
Ozone occurs when sunlight reacts with emissions in the air; it is the harmful ingredient in smog. On high ozone days, emergency room visits for respiratory problems can as much as double.
Particulate matter, made up of mainly carbon-based particles and dust.
Nitrogen dioxide forms when fuel is burned at high temperatures (as it is in on-road vehicles and power plants).
Sulfur dioxide is formed when fuel containing sulfur (mainly coal and oil) is burned during industrial pocesses.
Carbon monoxide, an odorless, colorless poisonous gas, comes mainly from motor vehicles and other exhaust.
Lead emissions are also dangerous pollutants, but have decreased significantly because Americans now use unleaded gasoline.